Norovirus — dubbed the ‘Ferrari of viruses’ because of how ferociously it spreads — is having its biggest surge in over a decade.
CDC data shows the infection, which causes violent diarrhea, vomiting and stomach pain caused nearly 100 outbreaks in the first week of December, the latest available, with more people sick at this time of year than since 2010.
One in four swabs for the virus also came back positive, data showed, almost double the level for the same time last year.
The explosion of norovirus is likely being driven by a newly-mutated variant of the virus.
But some also suggested that the surge could be linked to lockdowns, which left people unexposed to the common virus for several years, potentially weakening their immunity.
Other countries across the northern hemisphere are also reporting a surge, including the UK where cases have risen 40 percent on the same time last year.
It comes amid warnings over a quadruple-whammy of viruses hitting the US as millions return from the Christmas break, and amid concerns over a disease outbreak in China that has alarmed residents.
The above map shows reported norovirus outbreaks by state in the first week of December last year, the latest data available
Overall, data showed that the US recorded 91 norovirus outbreaks in the first week of December — 40 percent above the record for the same week from 2010 to 2024.
Across the US, Wisconsin reported the most outbreaks of norovirus — or where two or more cases were reported in hospitals or schools — at 22 in total.
Ohio (11), Virginia (6), California (5), Oregon (4) and Vermont(4) also recorded a higher number of outbreaks compared to other states.
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About 21million Americans are infected with the virus every year and, of these, around 2million are sent to their doctor's office or urgent care.
The disease is spread via contact with germs from an infected person's vomit or feces, contaminated food, shared utensils or surfaces they've touched.
While most people recover within a few days, the virus kills around 900 people every year, mostly adults 65 and up.
Outbreaks are most common in the winter due to a jump in the number of people gathering in groups for holiday festivities, traveling, and staying warm indoors, where pathogens can spread easily.
Symptoms of norovirus – nausea, vomiting, and diarrhea – come on quickly, within 12 to 48 hours of exposure.
Doctors say the illness can be avoided by thoroughly washing your hands with soap and water for at least 20 seconds, washing produce before eating it, sanitizing countertops, avoiding people who have been infected, and cooking seafood to at least 145 degrees.
The above graph shows the increase in positive norovirus tests from August 2024 through the first week of January 2025
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Data on outbreaks of norovirus was from CalciNet, the CDC's national norovirus surveillance network which tracks the virus nationwide using data from 34 labs in 29 states. Labs that are not part of the network can also submit data.
Separate data from the National Respiratory and Enteric Virus Surveillance System (NREVSS) — which receives data from 340 labs over 50 states — also revealed a surge in norovirus cases.
It showed 28 percent of swabs — or 828 out of nearly 3,000 tests done — detected the virus over the first week in January, equivalent to one in four detecting the virus.
This was up nine percent from the previous week, when 25.6 percent of swabs were positive, and 31 percent from two weeks ago, when 21.36 percent detected the virus.
By region, the data showed that Region 2 — which includes Missouri, Minnesota, Illinois, Michigan and Wisconsin — had the highest proportion of viruses testing positive at 29.89 percent.
For comparison, Region 1 — which is the Southern US — had the lowest proportion of positive tests at 14.63 percent detecting the virus.
Dr Lisa Lindesmith, an epidemiologist at the University of North Carolina, told Science: 'The early data for the early part of the season is certainly supporting that we're going to have a pretty intense norovirus year.'
Surveillance showed that a new variant called GII.17 — which has been present for years but at low levels — has surged recently.
Norovirus researcher Miranda de Graaf, from the Erasmus Medical Center in the Netherlands, says the variant found there has gained significant mutations.